More stringent fuel economy regulations in the transportation industry have prompted the need for improved efficiency of the IC engine. Light-weighting, friction reduction, thermal management, variable valve timing and a diverse array of variable valve lift (VVL) technologies are all part of the technology toolbox for IC engine designers.
VVL systems typically employ a technology in a valve train of an IC engine that allows different engine valve lifts to occur. The valve train consists of the components that are required to actuate an engine valve, including a camshaft, the valve, and all components that lie in between. VVL systems are typically divided into two categories: continuous variable and discrete variable. Continuous variable valve lift systems are capable of varying a valve lift from a design lift minimum to a design lift maximum to achieve any of several lift heights. Discrete variable valve lift systems are capable of switching between two or three distinct valve lifts. Components that enable these different valve lift modes are often called switchable valve train components. Typical two-step discrete valve lift systems switch between a full valve lift mode and a partial valve lift mode, often termed cam profile switching, or between a full valve lift mode and a no valve lift mode that facilitates deactivation of the valve. Valve deactivation can be applied in different ways. In the case of a four-valve-per-cylinder configuration (two intake+two exhaust), one of two intake valves can be deactivated. Deactivating only one of the two intake valves can provide for an increased swirl condition that enhances combustion of the air-fuel mixture. In another scenario, all of the intake and exhaust valves are deactivated for a selected cylinder which facilitates cylinder deactivation. On most engines, cylinder deactivation is applied to a fixed set of cylinders, when lightly loaded at steady-state speeds, to achieve the fuel economy of a smaller displacement engine. A lightly loaded engine running with a reduced amount of active cylinders requires a higher intake manifold pressure, and, thus, greater throttle plate opening, than an engine running with all of its cylinders in the active state. Given the lower intake restriction, throttling losses are reduced in the cylinder deactivation mode and the engine runs with greater efficiency. For those engines that deactivate half of the cylinders, it is typical in the engine industry to deactivate every other cylinder in the firing order to ensure smoothness of engine operation while in this mode. Deactivation also includes shutting off the fuel to the dormant cylinders. Reactivation of dormant cylinders occurs when the driver demands more power for acceleration. The smooth transition between normal and partial engine operation is achieved by controlling ignition timing, cam timing and throttle position, as managed by the engine control unit (ECU). Examples of switchable valve train components that serve as cylinder deactivation facilitators include roller lifters, pivot elements, rocker arms, roller finger followers, and camshafts; each of these components is able to switch from a full valve lift mode to a no valve lift mode. The switching of lifts occurs on the base circle or non-lift portion of the camshaft; therefore the time to switch from one mode to another is limited by the time that the camshaft is rotating through its base circle portion; more time for switching is available at lower engine speeds and less time is available at higher engine speeds. Maximum switching engine speeds are defined by whether there is enough time available on the base circle portion to fully actuate a locking mechanism to achieve the desired lift mode.
In today's IC engines, many of the switchable valve train components that enable valve deactivation for cylinder deactivation contain a coupling assembly that is actuated by an electro-hydraulic system. The electro-hydraulic system typically contains at least one solenoid valve within an array of oil galleries that manages engine oil pressure to either lock or unlock the coupling assembly within the switchable valve train component to enable a valve lift switching event. These types of electro-hydraulic systems require time within the combustion cycle to actuate the switchable valve train component.
In most IC engine applications, switchable valve train components for cylinder deactivation in an electro-hydraulic system are classified as “pressureless-locked”, which equates to:
a). In a no or low oil pressure condition, the spring-biased coupling assembly will be in a locked position, facilitating the function of a standard valve train component that translates rotary camshaft motion to linear valve motion; and,
b). In a condition in which engine oil pressure is delivered to the coupling assembly that exceeds the force of the coupling assembly bias spring, the coupling assembly will be displaced a given stroke to an unlocked position, facilitating valve deactivation where the rotary camshaft motion is not translated to the valve.
“Pressureless-unlocked” electro-hydraulic systems can be found in some cam profile switching systems that switch between a full valve lift and a partial valve lift, which equates to:
a). In a no or low oil pressure condition, the spring-biased coupling assembly will be in an unlocked position, facilitating a partial valve lift event; and,
b). In a condition in which engine oil pressure is delivered to the coupling assembly that exceeds the force of the coupling assembly bias spring, the coupling assembly will be displaced a given stroke to a locked position, facilitating a full valve lift event.
Switchable valve train systems often contain a lost motion spring or springs that provide a force during the unlocked mode to a component of the switchable valve train component assembly that is actuated by the camshaft, but does not translate rotary camshaft motion to linear valve lift. In many shaft-mounted switchable rocker arm systems, the lost motion spring is housed within a cylinder head or valve cover which can create packaging challenges. The lost motion spring provides a force that maintains contact between the actuated component and camshaft up to a maximum unlocked mode engine speed. FIGS. 15 and 16 show a prior art switchable rocker arm 100 for cylinder deactivation with a lost motion spring 150 that interfaces with the switchable rocker arm 100. A cam lever assembly 110 and a valve lever assembly 120 together form the switchable rocker arm 100. The cam lever assembly 110 is actuated by the camshaft 160 during the unlocked mode and interfaces with the lost motion spring 150 through a lost motion interface 190. With the shown position of the lost motion interface 190, a housing for the lost motion spring 150 is typically present above the switchable rocker arm 100, possibly in a valve cover or cylinder head cover (not shown). An alternative lost motion interface 190′ on the opposite end of the cam lever assembly 110 would also be possible, which would likely move the lost motion spring housing to a position below the switchable rocker arm 100 within the cylinder head (not shown). For both described locations of the lost motion spring 150, packaging space to house the lost motion spring 150 is required in an already packaging-challenged cylinder head environment of an internal combustion engine. Therefore, a switchable rocker arm with an integrated lost motion spring (or springs) that offers a smaller packaging space would be desirable.
While the cam lever assembly 110 is actuated by a full lift cam lobe 180 of a camshaft 160 during an unlocked mode, the valve lever assembly 120 remains stationary. For proper locking and unlocking of the cam lever assembly 110 to the valve lever assembly 120, rotational alignment of the two lever assemblies 110,120 and respective coupling assembly interfaces must be ensured during the base circle portion of the rotating camshaft. While rotational position and control of the cam lever assembly 110 is managed by the camshaft and lost motion spring 150 during the unlocked mode, proper rotational position of the valve lever assembly 120 is provided by an engine valve (not shown) at one end and a camshaft abutment 140 that interfaces with a zero-lift or base circle lobe 170 of the camshaft 160 on the opposite end. The camshaft abutment 140 can be especially helpful in switchable rocker arm designs, such as the one shown in FIGS. 14 and 15, that utilize a hydraulic lash adjuster 130 within the valve lever assembly 120. During the unlocked mode a pump-up condition can occur, in which the incoming oil pressure causes the hydraulic lash adjuster 130 to expand since it is not subjected to a normal valve actuation load. The camshaft abutment 140 can serve as a pump-up inhibitor, limiting the rotation of the valve lever assembly 120 due to pump-up of the hydraulic lash adjuster 130. However, the camshaft abutment 140 can be a source of undesirable friction and wear and requires the presence and corresponding cost of the base circle lobe 170 on the camshaft 160. Therefore, a switchable rocker arm that does not require the presence of a camshaft abutment and a corresponding base circle lobe on a camshaft would be desirable.
The packaging space required for the prior art switchable rocker arm 100 shown in FIGS. 15 and 16 must also take into account an arcuate lost motion of the cam lever assembly 110 during an unlocked mode as shown by the arrow within FIG. 16. In many cylinder head environments, this arcuate lost motion can lead to an interference condition with either the cylinder head itself or other assembled components within the cylinder head. Therefore, a switchable rocker arm that offers minimal lost motion packaging implications would be desirable.
Given the described packaging and corresponding cost challenges of implementing the prior art shaft-mounted switchable rocker arm within an IC engine, example embodiments will now be described that offer solutions for lost motion spring and arcuate lost motion packaging along with elimination of the camshaft abutment.